Gas-phase polymerization for the preparation of polyethylene can be carried out in a fluidized, or stirred, bed reactor in the presence of a catalyst, ethylene, fluidization gas and molecular weight regulator. In some instances, Ziegler-Natta catalysts are used.
For a given catalyst system, catalyst activity may depend on the polymerization conditions such as temperature and pressure. However, once the polymerization conditions are fixed, the activity depends on the catalyst system. When the activity is not satisfactory, the amount of catalyst fed to the reactor is increased or its residence time made longer.